This invention relates to an improvement in construction of flat glass plasma display panels, particularly those panels having conductors on a planar surface orthogonally arranged relative to a plurality of gas channels and separated therefrom by a dielectric material, wherein electrical energization of selected conductors causes gaseous ignition in a gas channel between the conductors, and permits the gaseous ignition to be controllably shifted along the gas channels. Because the electrical conductors are electrically insulated from the gas channels by means of a dielectric medium, panels of this type are characterized as AC plasma shift panels.
AC plasma shift panels have been disclosed in the prior art in different structural forms. For example, my U.S. Pat. No. 3,964,050 issued June 15, 1976, discloses a plurality of parallel conductors supported along a single planar substrate and having a plurality of gas channels orthogonally positioned relative thereto, with dielectric separation between the conductors and the gas channels. Adjacent conductors are connected to different electrical energization means, but every fourth conductor is connected to the same electrical energization means, so as to provide a repetitive electrical connection pattern wherein conductor 1, 5, 9, . . . is connected to voltage source A, conductor 2, 6, 10, . . . is connected to voltage source B, conductor 3, 7, 11, . . . is connected to voltage source C and conductor 4, 8, 12, . . . is connected to voltage source D. The voltage generated by sources A-D are time-phased so as to permit gaseous ignition between conductors 1 and 2, 2 and 3, 3 and 4, 4 and 5, etc. and thereby effectively shift the gas ignition over the respective conductors and along a gas channel. This type of voltage excitation, referred to herein as 4-phase excitation, provides a degree of voltage isolation so as to prevent spurious gaseous ignition between adjacently energized conductors, but suffers from the disadvantage that it requires four different voltage sources in order to shift the ignition along the gas channel. Such voltage isolation is critical in panels of this type, as the conductors are closely spaced in order to create a point ignition source and thereby provide maximum visual resolution of the images displayed on the panel. Conductors are typically spaced 0.005 inch-0.015 inch apart in panels of this type to obtain the visual resolution desired for good commercial quality. External connections to the four sets of planar conductors within the panel are made at two levels. The conductor ends of every fourth conductor are brought out and connected to a common bus line running along the panel on the same plane as the conductors themselves, i.e. the conductors associated with voltage source A; similarly, the conductor ends of all planar conductors connected to voltage source B are connected to a common bus line running along the side opposite the A bus, but also at the same planar level as the conductors. The planar conductors associated with voltage source C are connected via raised posts attached to each of the conductors to a bus line parallel to but at a raised planar level relative to the aforementioned two bus lines; finally, the planar conductors associated with voltage source D are similarly raised through conductive posts to a bus line at a raised planar level on the opposite side of the panel. In this manner, four independent electrical connections may be made to the plurality of parallel conductors, all of which conductors are along the same planar level in the panel.
In my U.S. Pat. No. 4,080,597, issued Mar. 21, 1978, there is disclosed another construction of an AC plasma shift panel wherein all conductors are on the same planar level, but are electrically driven by only three voltage power sources. This has the obvious advantage of requiring fewer voltage sources. The external electrical connections to the planar conductors are similar to that described in the preceding example, with the exception that only a single bus line is required at a raised planar level for connection to every third conductor through raised conductive posts. The voltage excitation of all planar conductors in this panel is accomplished by three time-phased voltage generators which provide the desired voltages for gaseous ignition and the relative timing of these voltages between planar conductors to permit shifting of the ignited gas along the gas channels.
Both of the aforementioned patents require extremely careful dimensional tolerancing in laying out the conductors on a substrate, expecially to achieve line spacing down to 0.005 inches, which is thought to be necessary for good visual resolution. Since each of the conductor ends to be connected through a raised conductor post to a bus bar along an elevated plane must have an enlarged conductive pad for making this connection, the dimensional spacing of these conductors is even more critical. Conductor line widths are typically 0.001-0.002 inches (1-2 mils), and conductor edge-to-edge spacing is typically 0.003-0.005 inches (3-5 mils) which means that an enlarged conductor pad must be carefully placed so as to avoid electrical contact with an adjacent conductor, and the method chosen for developing the raised conductive posts must be precisely located so as to exactly position the conductive posts atop the enlarged conductive pads without contacting adjacent conductors. The manufacturing process for accomplishing this is very precisely controlled in order that a working AC plasma shift panel may be constructed within the dimensional tolerances required.
There is an alternative construction for panels of this type wherein raised conductive posts are not used to bridge selected conductors to an elevated planar level. The alternative construction utilizes dielectric or insulating deposits over the conductive lines which are to be bridged by a crossing conductor. Lines which are to be conductively coupled do not receive the dielectric or insulating spot deposit, but subsequently a conductive line is laid between these conductively coupled lines and over the dielectric or insulating spot deposits. This alternative also creates difficulties when used in conjunction with closely-spaced lines, for there is a danger that the dielectric or insulating spot deposits will cover the lines which are intended to be conductively coupled.
In addition to the foregoing problems, it has been noted in the prior art that a very critical relationship exists between the planar conductor-to-conductor spacing and the depth of the gas channel bridging these conductors. Until now this relationship has been unknown, but it has been observed that, for a given conductor-to-conductor spacing, if the gas channel depth is made large spurious ignition tends to occur between energized conductors and conductors at some distance away along the same gas channel, and if channel depth is made too small ignition does not always reliably occur even between adjacent planar conductor pairs. This has been thought to be due to the electrical field pattern ganerated by the conductors, in combination with the mean free path of electrons in the gas, but the relationship has been generally undefined. In the foregoing two patents the preferred channel depth was selected through empirical procedures, without giving attention to planar conductor line spacing. It was known that performance of the panel could be influenced by proper choice of gas pressure and composition, and it was therefore believed that the relationship between planar conductor line spacing and channel depth was not critical.